The present invention relates to a central unit for a process control system, for instance a storage-programmable control system which consists of a central unit and a number of peripheral devices which are connected to the central unit by a bus system, the central unit having at least one control processor system with a control processor for the processing of real-time tasks as well as at least one additional remainder processor system which is separate from the control processor system and has a remainder processor for processing tasks which are not time-critical.
Processor control systems are widely used. As a rule, they have two tasks to fulfill. Namely, on the one hand, an application program must be processed in real time and on the other hand, the outside world, for instance, other process control systems, must be communicated with, programs must be loaded and peripheral devices must be communicated with.
The implementation of both of these tasks by the same central unit has up to now had the result that the two tasks have negatively influenced each other. In particular, the so-called alarm-reaction time was relatively long and furthermore not accurately reproducible due to the carrying out of two tasks. Since a part of the computing capacity had to be used for communication and management tasks, the theoretically possible alarm reaction time could not be reached.
European Patent Document No. EP-A-0496,097 relates to a process control system which has a central unit similar to that described above. However, even in this system, the carrying out of the two above-mentioned tasks cannot take place separately. Upon access by the communication processor to the common bus system, the running of the application program must namely be interrupted by a control processor, since there is only one common bus.